The present invention pertains generally to photodynamic therapy, as might be particularly useful in the treatment of cancers, viruses, and bacteria.
Photodynamic therapy (PDT) has been used widely in the treatment of a variety of cancers, including breast metastases, gynecological tumors, cutaneous cancers, Karposi""s sarcoma, and papillomatosis. In general, PDT has involved localization of a chromophoric dye molecule (e.g., porphyrins, chlorins, pheophorbides, and phthalocyanines) at a cancerous site, followed by optical excitation of the dye at relatively long wavelengths (e.g., xcex=650 nm or higher), where the transparency of human tissue is significant.
Particularly, the chromophore-containing dye molecule in its natural or ground state is a singlet (i.e., 1xcfx80xcfx80) such that the two electrons in the highest occupied molecular orbital are paired. Upon optical excitation, the photoexcited 1xcfx80xcfx80* state of the dye molecule decays non-radiatively to a triplet (i.e., 3xcfx80xcfx80*) state, which is lower in energy than the photoexcited singlet state of the dye molecule. In its triplet state, the dye molecule is then reacted with oxygen, which in its ground state is a triplet (i.e., 3O2). By way of a triplet-triplet annihilation mechanism, the excited triplet state of the dye molecule and the ground state (triplet) oxygen react to produce the ground state dye molecule and oxygen in the singlet state (i.e., 1O2), which is non-selectively cytotoxic. In this manner, existing PDT methods attempt to control the chemical decomposition of carcinogenic cells through selective optical initiation at wavelengths exhibiting moderate tissue penetration.
Photofrin(copyright), which is commercially available from QLT Phototherapeutics of Vancouver, British Columbia, is an example of a porphyrin type of compound (i.e., hematoporphyrin dimer) that utilizes this bimolecular triplet state mechanism to achieve photodynamic therapy. Indeed, Photofrin(copyright) has been used to treat esophageal, lung, bladder, gastric, and cervical cancers. In addition, some PDT studies have also been successful in treating viruses, such as papillomavirus, HIV, herpes simplex virus, measles and simian virus.
However, existing PDT approaches have been fraught with a number of disadvantages, primarily relating to the underlying bimolecular nature of these PDT approaches. In particular, existing PDT approaches have not been satisfactory for practical use in hypoxic environments because of the inherent reliance on the accessibility of oxygen. In addition, the fact that the cytotoxic process is based on freely diffusing singlet oxygen, 1O2, which is very reactive, is problematic because of the danger of generating unwanted reactions. Also, the effectiveness of PDT agents diminishes as photolysis times increase due to capillary collapse and a resulting restriction in blood flow, thereby causing reduced availability of O2. In this respect, the primary mechanism of existing bimolecular, oxygen-reliant PDT methods is vascular stasis, which halts the flow of oxygen and nutrients to tumor cells. As such, the lack of available oxygen inhibits the further effectiveness of the PDT agent to perform its function.
Apart from PDT, another class of anticancer agents pertain to calicheamicin and esperamicin enediynes. One feature of these systems is the unusual (Z)-1,5-diyne-3-ene unit that undergoes Bergman cyclization to produce a 1,4-benzenoid diradical. This species provides the thermodynamic driving force for the DNA-cleaving reaction by promoting H-atom abstraction from the deoxyribose ring. Formation of the diradical intermediate can be triggered by the presence of reducing agents such as NADPH or dithiothrietol.
Within this class, dynemicin-A is unique in that in addition to the reactive enediyne moiety, it also contains an anthraquinone chromophore that is responsible for the deep violet color of the molecule. The proposed mechanism of action of dynemicin-A suggests that reduction of the anthraquinone subunit induces epoxide ring opening, followed by tautomerization and Bergman cyclization of the enediyne linkage to produce a reactive benzene diradical intermediate that affords DNA-strand scission through H-atom abstraction. Unfortunately, these known enediynes have been ill-suited for use in in vivo treatments because of the high thermal reactivity and prominent synthetic challenges involved in obtaining the enediynes in sufficient quantities.
In addition to diradical-forming enediyne antibiotics, recent reports have shown that the terminal diazo-containing class of natural products including kinamycin C and prekinamycin are functional DNA-cleaving agents that utilize N2-release for activity. After their discovery, simple 9-diazofluorenes and other synthetic analogs have been prepared and shown very recently to induce thermal DNA-cleavage as well as photochemical strand scission upon UV excitation. Several different intermediates have been proposed to be involved in the DNA-cleavage process including both oxidative and reductive radicals as well as carbenes and a protonated form of 9-diazofluorene. However, existing terminal diazo compounds have not been fully satisfactory for use in in vivo treatments because they are simple organic compounds that do not have sufficiently long wavelength absorption features.
From the foregoing, it will be appreciated that there exists a need in the art for an approach for photodynamic therapy in which a chromophore is coupled to a photochemically reactive subunit that does not require oxygen as a coreagent. In this respect, there is a need for a photodynamic therapy approach that utilizes a unimolecular system that overcomes the drawbacks of oxygen-dependent photochemistry and functions in hypoxic environments without compromising the selectivity of optical initiation. There is also a need for a PDT approach in which sufficient quantities of desired compounds can be synthetically obtained readily and which is not subject to high thermal reactivity. It is an object of the present invention to provide such a photodynamic therapy approach that satisfies these needs. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The present invention provides novel compounds, compositions, and methods for photodynamic therapy which function by a unimolecular mechanism. Significantly, pursuant to the unimolecular mechanism, the inventive compounds, compositions, and methods do not require the presence of oxygen as a co-reagent to function in photodynamic therapy. Since it has been found that the compounds, compositions, and methods of the present invention perturb DNA (as described below), it is expected that the compounds, compositions, and methods of the present invention have significant utility in treating cancers as well as infections caused by microorganisms.
In accordance with the present invention, the compounds, compositions, and methods involve the formation of radical species for treating the cancers and/or infections. Desirably, the radicals are photochemically-induced (e.g., at visible wavelengths above 400 nm). In this respect, a therapeutically effective amount of a compound capable of forming a radical upon exposure to light by a unimolecular mechanism (e.g., in the absence of oxygen) is administered (e.g., by injection) to a patient so as to contact the cancer or microorganism selected for treatment. The compound is then irradiated at the site of action so as to induce radical formation. Strictly by way of example, the compounds can include metalloenediynes (i.e., transition metal complexes with metal chelating enediyne ligands) and/or transition metal complexes that bear at least one diazo functional group, such as, but not limited to, a terminal diazo group or as in a triazine (also generally referred to herein as xe2x80x9ctransition metal diazo compoundsxe2x80x9d or xe2x80x9ctransition metal diazo complexesxe2x80x9d).
Advantageously, the compounds, compositions, and methods of the present invention can be used in photodynamic therapy such that the presence of oxygen is not required in conjunction with light. As such, the compounds, compositions, and methods of the present invention can even function in hypoxic environments without compromising the selectivity of optical initiation. Also, desired compounds pursuant to the present invention exhibit high air and thermal stability, as well as high water solubility, which are particularly desirable attributes for photonucleases for photodynamic therapy applications. In addition, the inventive compounds, compositions, and methods also permit triggering of the photochemical reactivity in a controlled manner for biological applications. Furthermore, desired compounds can be synthetically obtained readily.
The present invention will be more fully understood upon reading the following detailed description in conjunction with the accompanying drawings.